This invention relates to a period-to-voltage converting device for producing a voltage corresponding to the period of a pulse signal (hereinafter referred to as "a period-voltage converter" when applicable).
When an electric motor is driven under constant speed control, in general a frequency proportional to the speed of the electric motor is produced and is converted into a DC voltage which is compared to the reference voltage, and the difference voltage between these two voltages is fed back to obtain the constant speed control. The frequency-to-voltage conversion can be achieved, for instance, in a method in which pulse counting is carrier out. However, in this method the voltage obtained by the conversion contains not only DC components but also ripple components, as a result of which the electric motor is driven also by the ripple components. Therefore, if this method is employed for an audio turntable motor, then the S/N ratio will be considerably decreased.
In order to overcome the above-described difficulty, a method is often employed in which the period is converted into voltage according to a sample-and-hold system, which is less affected by the ripple components. One example of such a period-voltage converter is shown in FIG. 1, and various waveforms in the sections of the converter are indicated in FIG. 2. An input signal is applied by a first monostable multivibrator 1 to obtain sampling pulses having a small pulse width which are applied to a second monostable multivibrator 2 to provide charging pulses. The charging pulses are applied to a first gate circuit 3 to open the latter thereby charging a first capacitor 6 to a predetermined voltage. When the charging pulse is not applied, the capacitor 6 starts discharging with the aid of a constant current source 7, as a result of which a saw tooth wave is produced. When the next sampling pulse is produced, a second gate 4 is open, and the voltage of the capacitor 6 is sampled by a second capacitor 8. Thus, a voltage substantially corresponding to the period can be obtained.
It is true that in this mehtod a voltage substantially corresponding to the period can be relatively readily obtained. However, this method still suffers from disadvantages that the pulse width of the sampling pulse and that of the charging pulse are varied with the variation in time constant of the monostable multivibrators which is caused by variation in temperature. This variation in time constant affects the output voltage and lowers the stability. Furthermore, since a capacitor is required for forming the monostable multivibrator, it is necessary to externally mount the capacitor where the converter is manufactured in the form of an integrated circuit.